1) Field of the Invention
The invention relates to the preparation of a graft copolymer latex of core/shell dispersion particles having improved phase binding between core and shell in a two-stage emulsion polymerization process, dispersions and graft copolymers prepared by the process and the use thereof.
2) Background Art
The preparation of particulate graft polymers with the aid of two-stage emulsion polymerization is known. In this process, a polymer dispersion is produced in a first stage with the aid of emulsion polymerization and then, in the second stage, a shell of another polymer is grafted onto said polymer dispersion by metering in the conventional initiators and further monomers. This process is also used, inter alia, for grafting elastomeric polymer dispersion particles, which are characterized in particular by a glass transition temperature of less than 0.degree. C., with hard monomers. Such a procedure is described in DE-A 3743142 for the graft polymerization of vinyl chloride onto an ethylene/vinyl acetate rubber. In the preparation of all these particulate graft polymers synthesized in two-stage processes, the main problem is the binding of the graft shell to the grafting base. In the case of insufficient phase binding, the mechanical properties of the corresponding moldings are in fact generally unsatisfactory. The prior art has already disclosed a number of measures for improving the phase binding.
EP-A-231933 describes the use of so-called graftlinking monomers. By definition, these are polyfunctional monomers having a plurality of C.dbd.C double bonds of different reactivities, not all of which react in the free radical polymerization of the first stage and some of which are therefore available in the second stage for binding to the monomers to be grafted. The disadvantage here is that some of these polyfunctional monomers also act as crosslinking agents in the first polymerization stage. In addition, it is frequently observed that the less reactive double bonds have a retardant effect in the first reaction stage. Consequently, these graft-linking monomers cannot be introduced in any desired amounts into the grafting base.
The use of hydroxyalkyl acrylates as comonomers in the grafting base for improving the grafting rate is claimed in DE-A 4006643. The disadvantage of this process is that, owing to their copolymerization parameters, the hydroxyalkyl acrylates proposed there cannot be copolymerized with all vinyl monomers in the first stage.
The use of copolymerizable peroxide initiators which are copolymerized in the first stage together with the monomers of the grafting base to give a so-called macroinitiator which can initiate a graft polymerization in the second stage without further addition of initiator has already been described many times in the patent literature, for example in DE-A 4135984 and in the literature cited therein. The disadvantage of all these processes is the expense in the copolymerization in the first stage to give the macroinitiator, in which care must be taken to ensure that the peroxide functions are not destroyed.
In the process of DE-A 4000543, the "in situ" generation of a particulate macroinitiator having hydroperoxide groups at the particle surface is claimed. For this purpose, a grafting base produced in emulsion is treated with water-soluble peroxides and atmospheric oxygen. According to this publication, hydroperoxide groups form on the particle surface, which groups can be activated in the grafting step of the second stage with the aid of a reducing agent and finally act as anchor groups for the polymer shell grafted on. The disadvantage of this process is the use of molecular oxygen, which, owing to its diradical character, is known to have a retardant effect in free radical reactions and therefore must be removed by an expensive procedure before the subsequent graft polymerization.
It was therefore the object of the invention to develop a process for the preparation of particulate graft copolymers having improved binding between the grafting base and the graft shell, by means of which the disadvantages of the abovementioned procedures can be avoided.